Impregnants for dispenser cathodes

ABSTRACT

A dispenser cathode suitable for use as an electron emitter for microwave tubes and the like. The dispenser cathode is constituted by a porous body of refractory material such as tungsten, impregnated with a fused mixture of barium oxide, and at least one oxide of a rare earth metal, such as holmium oxide, terbium oxide, thulium oxide or yttrium oxide.

United States Patent [191 Koppius March 6, 1973 IMPREGNAN TS FORDISPENSER CATHODES [76] Inventor: Otto G. Koppius, PO. Box 187, Highways27 & 50, Clermont, Fla. 32711 22 Filed: May 19,1971

21 App1.No.: 144,916

[52] US. Cl "313/346 DC, 313/311, 313/346 R [51] Int. Cl ..H01j 1/14,HOlj 19/06 [58] Field of Search ..313/346 R, 346 DC, 311

[56] References Cited UNITED STATES PATENTS 3,358,178 12/1967 Figner eta1 ..3l3/346R 2,813,807 11/1957 Levi ..313/346 R X 3,437,865 4/1969Gabor et a1. ..313/346 R 3,558,966 l/l971 Hill et al. ..313/346 RPrimary Examiner-David Schonberg Assistant Examiner-Toby H. KusmerAttorney-Michael Ebert 5 7] ABSTRACT 4 Claims, 7 Drawing FiguresIMPREGNANTS FOR DISPENSER CATHODES BACKGROUND OF THE INVENTION Thepresent invention relates generally to dispenser cathode structures, tomethods for fabricating such structures and to improved impregnantstherefor.

Dispenser cathodes are known in which a porous body of a refractorymetal such as tungsten is impregnated with a supply of an alkaline earthcomposition capable of furnishing free alkaline earth metal to thecathode surface. In the US. Pats. Nos. 2,700,000, 2,813,807, 3,118,080and 3,201,639, there are disclosed dispenser cathodes in which apreformed porous body of refractory metal is impregnated from a meltwith a fused mixture of barium oxide and one or more other metal oxides.During operation of the cathode, the mixture reacts with the refractorymetal to supply free barium to an emissive surface of the cathode.

The refractory metal body of such cathodes may be formed by compressinga finely divided powdered metal to form a green body and heating thebody to an elevated temperature at which the particles sinter togetherand form a cohesive mass which is dense yet porous. To work this dense,hard, porous mass of refractory material into intricate bodies suitablefor commercial microwave tube structures adapted to deliver moderate tohigh power in the megawatt range, I prefer to use the techniquedescribed in my prior US. Pat. No. 3,538,570.

The pores in such a body thus obtained may now be filled with analkaline earth composition. The alkaline earth metal composition isfirst heated to a temperature at which the composition melts in aneutral or reducing atmosphere. The porous body is then brought intointimate contact with the molten material, as by immersing the porousbody in the molten material, or by placing the material on the body andconverting it to a molten state. While in the molten state, the materialflows or diffuses into the pores of the body and completely fills allvoids within the porous body.

After cooling, the body is mounted in an evacuated envelope andactivated by heating to a temperature at which the alkaline earthcomposition reacts with the refractory metal to supply alkaline earthmetal which covers the surface of the body. The cathode is now ready foroperation as an electron emissive device.

As the refractory metal, tungsten is preferred, but molybdenum, hafnium,tantalum, niobium, and rhenium may be used.

The alkaline earth metal composition is the most critical material forthe formation of high quality electron emitting cathode structures. Theexact manner in which each component of the alkaline earth metalcomposition plays in the role of an electron emitter is unknown.Presently in widespread commercial use are dispenser cathodes of thetype disclosed in the Levi US. Pat. No. 3,201,689 wherein the impregnantis a fused mixture of barium oxide and at least one oxide selected fromthe group consisting of aluminum oxide and boron oxide.

SUMMARY OF THE INVENTION The main object of the invention is to providea dispenser cathode whose impregnant is constituted by a compositioncontaining barium oxide and an oxide of a rare earth element.

Quite unexpectedly, l have found that impregnant compositions containingbarium oxide and the oxides of the rare earth elements, producedispenser cathodes which are exceptionally good electron emitters andwhich perform better than those which are now commercially available.

The chemical elements grouped with lanthanum in the periodic tablenumbers 57 through 71, are traditionally referred to as rare earthelements. In recent years, the term, rare earth elements has beenextended to include two outlying elements, namely scandium (element 21)and yttrium (element 39), which fall in the same group (IIIB) of theperiodic table. The term rare earth is really a misnomer, for theelements in the group are not rare as compared to gold or platinum, andthey are not earth, but true metal.

In order of increasing atomic number, the rare earths are scandium(symbol Sc); yttrium (Y); lanthanum (La); cerium (Ce); praseodymium(Pr); neodymium (Nd); promethium (Pm); samarium (Sm); europium (Eu);gadolinium (Gd); terbium (Tb); dysprosium (Dy); holmium (Ho); erbium(Er); thulium (Tm); ytterbium (Yb); and lutetium (Lu).

More specifically, it is an object of the invention to provide animproved impregnant for a dispenser cathode which consists of a fusedmixture of barium oxide and an oxide of a rare earth metal.

Briefly stated, these objects are attained in a thermionic dispensercathode, comprising a porous body of refractory material, preferablytungsten, having at least one surface portion adapted to afford anelectron emission surface provided with a large number of smallpassageways connecting the interior of said body to said surface.

Dispersed within the body is an impregnant constituted by a fusedmixture of barium oxide and at least one rare earth oxide in a moleratio providing relatively high emission.

OUTLINE OF THE DRAWING For a better understanding of the invention aswell as other objects and further features thereof, reference is made tothe following detailed description to be read in conjunction with theaccompanying drawing, wherein:

FIG. 1 is a graph showing the emission of a dispenser cathode whoseimpregnant includes holmium oxide;

FIG. 2 is a graph showing the emission of a dispenser cathode whoseimpregnant includes terbium oxide;

FIG. 3 is a graph showing the emission of a dispenser cathode whoseimpregnant includes thulium oxide;

FIG. 4 is a graph showing the emission of a dispenser cathode whoseimpregnant includes scandium oxide and yttrium oxide;

FIG. 5 is a graph showing the emission of a dispenser cathode whoseimpregnant includes calcium oxide and yttrium oxide;

FIG. 6 is a graph showing the emission of a dispenser cathode whoseimpregnant includes scandium oxide; calcium oxide and yttrium oxide; and

FIG. 7 is a plot illustrative of the emission test procedure.

DESCRIPTION OF THE INVENTION Promethium and Lutetium are notcommercially produced in any quantity and have not therefore, beentested in cathode structures. The commercially available rare earthoxides which were tested are Cerium, Praseodymium, Neodymium, Samarium,Europium, Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, andYtterbium. The oxides of Yttrium and Scandium were tested as carriersfor barium oxide in cathodes.

Each of the above-mentioned impregnants were used as an electronemissive material.

A small amount of the impregnant material was placed on a test cathodeporous tungsten body and impregnated by melting as described previously.The theoretical density of the porous tungsten body was 82 percent inall tests. The impregnation time was 2 minutes. Such test cathodes weremounted in a suitable diode structure, evacuated, and tested forelectron emission capabilities as will be described later.

evaporation. The emission curves are shown in FIGS. 1, 2, and 3.

FIG. 1 is a graph showing the emission of a dispenser cathode impregnantwhich is a fused mixture of 3 moles Barium oxide and 1 mole Holmiumoxide; FIG. 2 is a graph for an impregnant which is a fused mixture of 3moles Barium Oxide and 1 mole Terbium oxide; FIG. 3

is a graph for an impregnants which is a fused mixture of 3 moles Bariumoxide and 1 mole thulium oxide.

The zero field emission at l,000 C Br (Brightness) was found to be 3.3,2.9 and 2.6 amps/cm respectively for the three different impregnants.The melting point of the impregnants are l,790 C, l,750 C, and l,800 Crespectively. Test cathodes made with Scandium oxide gave high emission,good life, but the amount of barium evaporation was much too high. Themelting point of 3 BaO: 1 Sc,O composition was l,7l C. Cathodes madefrom Yttrium oxide gave good emission, low barium evaporation, but weredifficult to impregnate. The melting point of 3 BaO: IY,O mixture was1,950 C. Such a high melting point complicates other procedures whichmake a cathode attractive commercially. Many cathode structures need apotted heater and the high melting point of the impregnation step causesundue shrinkage of the heater potting material.

A combination of barium oxide, scandium oxide and 6 Barium oxide 1Scandium oxide 1 Yttrium oxide.

The melting point was found to be 1,790 C. The impregnation time was 2minutes. The zero field emission was 3.4, as shown in the graph in FIG.4, and the amount of metallic barium evaporation was moderate.

A combination of barium oxide, calcium oxide, and yttrium oxide was madeand tested. The mole combination was:

6 Barium oxide 1 Calcium oxide 1 Yttrium oxide.

The melting point was found to be l,730 C. The impregnation time was 2minutes. The zero field emission was 2.9, as indicated by the graph inFIG. 5 and the amount of metallic barium evaporation was very low. It isto be noted that each of these impregnants has a zero field emissionbetter than the one now used commercially, the published value of whichis 2.5 amps/cm at l,000 C Br. Further it is to be noted that theaddition of scandium oxide and/or calcium oxide reduced the meltingpoint of the barium oxide/yttrium oxide combination to values that arepractical, i.e., to a melting point of about 1 ,800 C or lower.

A combination of barium oxide, calcium oxide, scandium oxide and yttriumoxide gave surprising results. The optimum mole ratio combination wasfound to be:

6 E210 (barium oxide) isc o (scandium oxide) fzCaO (calcium oxide) 1 Y o(yttrium oxide) i.e., 6 BaO; %Sc O :%CaO: l Y O where the number ofmoles of barium oxide can vary between S and 7. The melting point ofthese combinations are 7 1 ,740 C; l,680 C and l,690 C for 5,6, and 7moles of barium oxide, respectively, with the other components remainingfixed. t

The impregnation time can vary between 30 sec; and 2 min. with 1 minutebeing optimum for full impregnation. The melting point of all threecombinations is well within practical limits for the performance ofother procedures that must be accomplished for producing commerciallyacceptable cathodes. Additionally, I was surprised to find that cathodesmade with this impregnant gave unusually high zero field emission ofabout 5 amps/cm at l,000 C. In addition, the evaporation of metallicbarium from the cathode is very low and the emission life is-in excessof 2,000 hours.

The graph in FIG. 6 shows the pulsed emission test data of the newimpregnant as compared to a commercially used type containing bariumoxide, aluminum oxide, and calcium oxide in about a mole ratio 52:3. Thezero field emission of the new impregnant is about a factor of twobetter.

In addition, there is one other surprising difference between the twoimpregnants. The pulsed electron emission of the new impregnant has avery much higher slope at high field strength than the other. W isproportional to Field Strength). The performance of the new cathodeimpregnant was found to be much better in devices which require a highfield strength at the cathode. Most magnetron tubes and some ion gaslaser tubes require such a cathode characteristic for maxiinumperformance.

I shall now describe the emission procedures developed in connectionwith my new impregnants.

Conventional planar test diodes were made to determine the emissioncharacteristics of the new electron emissive impregnants. Also,conventional Schottky plots were made for each cathode under pulsedconditions. The zero field emission at any given temperature representsa figure of merit of a cathode. It is a value whereby one can compareperformance data. The method by which it is determined will be givenpresently.

The diodes were simple glass bulbs containing a molybdenum plate and thetest cathode along with a suitable cathode electrical heater. These weremounted on a glass stem having molybdenum feed-through leads. A bariumgetter was positioned on one lead in such a way that when flashed itwould not interfere with the tests. The plate-to-cathode spacing was0.75 mm. This device was thoroughly evacuated, baked, and sealed fromthe pumping system. Life tests of the cathode were run at 1,050 C Br.with sufficient plate voltage to run the plate red hot. Evaporation ofmetallic barium could be observed with time from a darkening of theglass bulb. Also emission data could be taken as a function of time andtemperature.

The standard Schottky equation relates the emission from the cathodewith the applied plate voltage (or Field). The Schottky equation equatesthe Richardson emission equation with the voltage or field enhancedemission. For high plate voltages and for a fixed temperature theequation approaches a straight-line equation of the form y a bx; where bis the slope and a is the y intercept when x 0. The Schottky equationbecomes:

logl=A+B V1 stant for a fixed value of the cathode temperature. I is theemission current in amperes/cm Thus an extrapolation of the straightline portion to zero plate voltage gives the intercept A, which valuerepresents the emission of the cathode at zero field (plate voltage). Atypical plot has the appearance shown in the graph illustrated in FIG.7. The Schottky equation is:

Constant A Constant B (At constant Tc) I =amperes/cm.

log I: A+BW where Field=Vv/d in amperes/cm While there have been shownand described preferred embodiments of impregnants for dispensercathodes in accordance with the invention, it will be appreciated thatmany changes and modifications may be made therein without, however,departing from the essential spirit of the invention.

I claim:

1. A thermionic dispenser cathode comprising a porous body of refractorymetal having at least one surface position adapted to afford anelectron-emission surface, and provided with a large number of smallpassageways connecting the interior of the body to said surface, and animpregnant disposed within said body and constituted by a fused mixtureof barium oxide, yttrium oxide and at least one rare earth oxideselected from the class consisting of calcium oxide and scandium oxidein a mole ratio providing relatively high emission.

2. A cathode as set forth in claim 1, wherein said fused mixtureconsists essentially of 6 moles barium oxide, 1 mole calcium oxide, and1 mole yttrium oxide.

3. A cathode as set forth in claim 1, wherein said fused mixtureconsists essentially of 6 moles barium oxide, 1 mole scandium oxide, and1 mole yttrium oxide.

4. A cathode as set forth in claim 1, wherein said fused mixtureconsists essentially of 6 moles barium oxide, one-half mole scandiumoxide, one-half mole calcium oxide, and 1 mole yttrium oxide.

1. A thermionic dispenser cathode comprising a porous body of refractorymetal having at least one surface position adapted to afford anelectron-emission surface, and provided with a large number of smallpassageways connecting the interior of the body to said surface, and animpregnant disposed within said body and constituted by a fused mixtureof barium oxide, yttrium oxide and at least one rare earth oxideselected from the class consisting of calcium oxide and scandium oxidein a mole ratio providing relatively high emission.
 2. A cathode as setforth in claim 1, wherein said fused mixture consists essentially of 6moles barium oxide, 1 mole calcium oxide, and 1 mole yttrium oxide.
 3. Acathode as set forth in claim 1, wherein said fused mixture consistsessentially of 6 moles barium oxide, 1 mole scandium oxide, and 1 moleyttrium oxide.